The present invention relates to a process for treating steel plates for use as offset printing plates. The process produces an anti-corrosive effect on the plates, in addition to uniformly roughening the plate surface. The invention also relates to an electrolyte solution useful in the process.
Offset printing plates which, for simplicity, are hereinafter referred to as printing plates, are generally composed of a support to which as least one radiation-sensitive reproduction layer is applied. The reproduction layer is applied to the support either by the user, in the case of plates which have not been precoated, or by the industrial manufacturer in the case of precoated (presensitized) plates.
The printing plate supports predominantly used comprise metallic materials, principally aluminum and its alloys. However, support plates of normal carbon steel or steel alloys, for example, chrome-nickel steels, manganese steels and the like, are also used.
In order to obtain certain necessary printing plate properties, such as an adhesive capacity for the layer, differentiation of hydrophilic and hydrophobic areas with defined behavior, corrosion properties and surface hardness, which is important for the length of the printing run, the printing plate is, in general, subjected to a pretreatment. This pretreatment includes, for example, a modification by mechanical, chemical or electrochemical roughening, which is also referred to as graining or etching, chemical or electrochemical surface oxidation, treatment with agents which confer hydrophilic properties.
A combination of the above types of modification is frequently used in the modern, largely continuously operating high-speed units of the manufacturers of uncoated or precoated printing plates.
When aluminum or aluminum alloys are used, the modification comprises, in most cases, a combination of mechanical and/or electrochemical roughening and an anodic oxidation, followed, if appropriate, by a stage in which the plate is rendered hydrophilic.
Aluminum-based carrier plates, due to the material, are widely used and have proven largely satisfactory, even though they have a lower mechanical strength and wear resistance than steel plates. However, the aluminum-based support plates are not amenable to advantageous magnetic fixing to the printing cylinders. The desirable property of magnetic fixing is of particular interest for high-speed rotary presses.
In order to eliminate, in particular, this disadvantage of the aluminum-based printing plates, printing plate support formed as sandwich plates, for example, are not being used for certain applications.
For example, German Offenlegungsschrift No. 2,544,295 discloses sandwich plates composed of a base support of aluminum or steel, on which printing and non-printing areas comprising two different metals are present. The printing areas mainly comprise copper, and the non-printing areas comprise chromium. Such sandwich plates are advantageous with respect to magnetic fixing and exhibit strength, buckling resistance and surface hardness. A general disadvantage of the sandwich plates, however, among other things, is their technically complicated manufacture. For the preparation of the layers, precisely adjusted electroplating baths are required, the disposal of which involves effluent problems and many of which also have high energy consumptions. Moreover, adhesion promoters must be applied as intermediate layers in order to ensure adhesive strength of the individual layers to one another and to the base support material. These procedures are expensive in technical implementation. An additional factor contributing to the expense and technical difficulty is that the electrolytes contain multi-component mixtures which must be mutually matched with great accuracy. Moreover, in the case of improper storage and/or development, there is also a certain risk of the layer adhesion being partially loosened.
In order to avoid the demonstrated disadvantages of the sandwich plates, a steel-based printing plate has been developed according to German Offenlegungsschrift No. 3,100,630. Steel as a lithographic support material is sufficiently hydrophilic for a direct formation of non-image area regions; however, it has the disadvantage of being very susceptible to corrosion. In order to provide the plate with corrosion protection, the steel plates are treated with an inhibiting salt solution, for example, a sodium nitrite solution, after the electrochemical roughening in a chloride solution. Subsequently, the light-sensitive coating is applied. The agents used for conferring hydrophilic properties are hexacyanoferrates or hexacyanocobaltates. Compared with the above-mentioned printing plates, these printing plates have the advantage of strength, buckling resistance, magnetic adhesion in the printing presses and a certain corrosion resistance upon storage and/or upon development of the exposed plates and/or during the printing process.
A serious disadvantage of the plates appears, however, in the roughening stage. Depending on the steel grade used (manufacture, composition), the roughening is not sufficiently uniform, as desired for a printing plate support, especially in view of the adhesive strength of the light-sensitive coating which is to be applied. The evaluation by measurement of peak-to-valley heights shows extensive non-uniformity of the plates described above, and especially the formation of so-called pits is to be noted as an unfavorable surface property. The formation of scars results from pitting corrosion mainly on existing defects in the starting material. This produces unfavorable and unavoidable results with respect to the coating and/or development of the plate and hence finally the quality of the later printed image.